In conventional electric glass melting furnaces, a pool of molten glass is heated by side or bottom entering electrodes to melt particulate glass batch present as a thick layer on top of the molten glass pool. The undersurface of the batch layer or blanket is melted by its contact with the molten pool as additional unmelted batch is added to the top of the batch blanket. The batch blanket typically is at least about 2 to 3 inches thick and serves to insulate the top of the molten pool from the ambient atmosphere.
The melting rate of a conventional electric melting furnace generally has been controlled by varying the amount of heating current to the electrodes in accordance with the level of molten glass in the furnace or forehearth, thus varying the temperature of the molten glass pool beneath the batch blanket. The glass level, i.e., the location of the top of the molten glass pool, will fluctuate in accordance with variations in the temperature of the molten glass pool, since hotter glass will melt more glass from the batch blanket to raise the glass level.
In some instances, such as in direct melt fiberglass melting furnaces where the furnace output is directly connected to the fiber-forming bushing, the throughput of the furnace and the hydrostatic or glass head over the bushing must be maintained substantially constant in order to prevent undesirable fluctuations in the fiber-forming operation, e.g., variations in the fiber diameter and the yardage produced. To maintain the throughput substantially constant, the depth of the molten glass pool must be maintained constant, and various forms of glass level controls have been utilized. Infrared detectors, ultrasonic detectors, glass level probes and other level detecting mechanisms have been tried, but accurate glass level detection and control is rendered difficult by the presence of the relatively light, thermal insulating, particulate glass batch blanket on top of the molten glass pool. Other difficulties have arisen from the attempted correlation of level control with the glass temperature control by varying the current supply to the electrodes.
As a result, the prior art has not provided a satisfactory means for maintaining substantially constant molten glass throughput from a glass melting furnace.